This invention relates to a magnetic refrigerator utilizing the magnetocaloric effect.
A magnetic refrigerator of this type is adapted to condense a gas with a magnetic substance which is cooled by adiabatic demagnetization. This magnetic refrigerator has an excellent refrigeration power per unit volume over an ordinary compression type refrigerator.
The magnetic refrigerator requires two alternate heat exchange processes: a heat elimination process for quickly moving a magnetic substance (a working substance), such as a gadolinium-gallium-garnet structure, into a magnetic field to permit it to be adiabatically magnetized and for causing heat generated in the working substance to be dissipated towards the outside and a heat absorption process for quickly moving the working substance away from the magnetic field to cause it to be adiabatically demagnetized so that a gas may be condensed by resultant heat absorption.
Therefore, the refrigeration coefficient and power of the magnetic refrigerator are greatly influenced by a heat exchange coefficient at the adiabatic magnetization time (heat dissipation at high temperature) and adiabatic demagnetization time (heat absorption at low temperature). In order to enhance the refrigeration coefficient and power, it has been necessary to eliminate heat, which is generated in the working substance at the time of heat elimination at high temperature, as quickly as possible to pre-cool the working substance to a required low temperature, or it has been necessary to condense a greater amount of gas at a time of heat absorption at low temperature. Therefore, a trade-off is involved therebetween, if viewed from the standpoint of the working substance. That is, a working substance of a greater area is required if a greater amount of gas at a time is condensed at the time of heat absorption at low temperature. However, a gadolinium-gallium-garnet structure, a typical working substance, is a single crystal and it is generally difficult to obtain a greater-diameter and greater-area single crystal. It is therefore necessary to provide an uneven surface to a smaller-diameter and smaller-area working substance and thus to increase the surface area. In order to improve the heat exchange efficiency at the time of heat elimination at high temperature, it is required that a heat conductor for heat dissipation be placed in intimate contact with the working substance. For a working substance of an uneven surface, however, difficulty is encountered in placing it in intimate contact with such a heat conductor. It is therefore impossible to satisfy two such demands simultaneously. In this magnetic refrigerator, an improvement is desired to lower the refrigeration efficiency and power.